1. Field of the Invention
The present invention is directed to ventilator systems which provide respiratory gas for artificial respiration of a patient. More particularly, the present invention is directed to volume ventilator systems which have improved control over the flow rate and pressure parameters of the respiratory gas which is made available to the patient.
2. Brief Description of the Prior Art
Artificial ventilators, or ventilator systems which provide air, or air and oxygen mixtures for artificial respiration by a patient, are well known in the art.
One type of artificial ventilator of the prior art is generally known as a "pressure ventilator", because, in operation, such ventilators make the respiratory gas available to the patient in accordance with a substantially predetermined pressure-versus-time cycle.
Another type of artificial ventilator of the prior art is known as a "volume ventilator", because such ventilators make a predetermined volume of respiratory gas available to the patient in each breathing cycle. In other words, "volume ventilators" deliver the respiratory gas to the patient in accordance with a predetermined volume-versus-time, or flow rate-versus-time function.
As is well understood by those skilled in the art, an artificial ventilator should, ideally, be capable of accurately monitoring several parameters of the artificial ventilation process, and of reliably maintaining those parameters within predetermined limits. More specifically, the parameters which should be accurately and reliably maintained include the percentage of oxygen in the respiratory gas (when a mixture of air and oxygen, rather than just air, is prescribed for the patient), the flow rate of the respiratory gas to the patient, and the pressure of the gas in the system which is proximal to the patient (proximal pressure).
As noted above, a volume ventilator operates on the basic principle that a predetermined volume of respiratory gas is delivered to the patient in each breathing cycle. The flow rate of the gas, however, is not necessarily constant during the cycle. Rather, the respiratory gas is often delivered to the patient in accordance with a predetermined flow-rate-versus-time (or, what is essentially an equivalent, predetermined volume-versus-time) curve, determined by a physician in a prescription tailored to the individual requirements of the patient.
The proximal pressure of the respiratory gas (like the flow rate) is usually also not kept constant in prior art ventilators. Rather, during certain modes of ventilation the proximal pressure is usually controlled by an exhalation valve which opens and closes to maintain a predetermined pressure level in the system. However, the pressure level maintained by the exhalation valve is not constant during the ventilation cycle. Rather, it varies in a time cycle to permit inflation of the patient's lungs with respiratory gas, and thereafter to permit deflation down to a predetermined pressure level, while the patient exhales. Furthermore, the exhalation valve also functions as an important safety valve, to minimize the possibility of accidental "overpressurization" of the patient.
In addition to the foregoing briefly summarized requirements, artificial ventilators must, or at least ideally should, operate very reliably and safely. Safety, for example, requires not only accurate maintenance of the control parameters of the ventilation process, but also rapid access to ambient air to allow the natural breathing of the patient when an electrical power failure or other serious malfunction occurs in the system.
Still further, artificial volume ventilators ideally should be able to deliver several types of breaths, such as "volume controlled" breath where the ventilator entirely provides the breathing effort of the patient (briefly described above), and "assisted volume controlled" breath and "spontaneous" breath. In the latter two types of breath the ventilator detects and assists or supports the spontaneous breathing efforts of the patient.
The prior art has developed several ventilators in efforts to more-or-less satisfy the above-noted and other requirements. Typically, prior art volume ventilators include a blender for providing a mixture of air and oxygen of a predetermined concentration, a computer assisted flow control subsystem, and a computer assisted pressure control subsystem. The flow control subsystems of the prior art usually comprise a positive displacement piston, the movement of which is directed by a computer in accordance with a predetermined volume-(or flow) versus-time function. The pressure control subsystems of the prior art volume ventilators typically comprise a pressure regulator which outputs a pilot pressure to pneumatically control the release pressure of the exhalation valve.
Examples of prior art artifical ventilators and of specific components of such ventilators, or related systems can be found in U.S. Pat. Nos. 4,036,221; 4,177,830; 4,336,590; 4,326,513; 4,204,536; 3,903,881; 4,190,045; 4,448,192; 4,323,064; 4,262,689; 4,333,453; 4,097,786.
Although prior art ventilators have proven suitable for many clinical needs, the need has been recognized for increasingly accurately monitoring and maintaining the time cycled flow rate and proximal pressure parameters of the ventilators. Therefore, the present invention, which provides a ventilator of improved ability to accurately and reliably maintain the foregoing important parameters, represents a major step forward in the art.